Turkey backs Syrian rebel factions who control part of the north and has observation posts across rebel-held Idlib province as part of a "de-escalation" agreement signed in 2017 with Assad's ally Russia, and Iran.

Ankara has threatened for months to launch a new offensive in northern Syria to drive out US-backed Syrian Kurdish fighters.

It has twice led incursions into northern Syria in 2016 and 2018. Since then, its forces and allied Syrian proxies have controlled a patch of territory on the border.

Ankara has repeatedly threatened to march on areas further east, where the US-allied fighters of the Kurdish People's Protection Units (YPG) are in control.

The YPG is seen as an effective ground force by the US in the fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group, but Turkey says it is linked to the outlawed Kurdistan Workers' Party (PKK), which Ankara and Washington list as a terrorist group.

Washington last month said it would pull its 2,000 troops from the country, leaving the Kurds scrambling to find a new ally in Damascus to avoid a potential Turkish assault.

On Friday, Erdogan said Turkey expected a "security zone" to be created in Syria in a few months.

The Adana deal was signed in 1998 to end a crisis between the neighbours, sparked by the presence in Syria of PKK leader Abdullah Ocalan and bases run by the group.

Turkey argues the protocol provides Ankara with the legal ground to intervene in Syria against the PKK and its affiliates, because of the Syrian regime's failure to act against the group.

Damascus has regained control of almost two-thirds of the country after Russia intervened militarily in 2015.